1. Field of the invention
The present invention concerns wave propagation structures for eliminating voltage surges and absorbing transients.
2. Description of the prior art
Electronic components exhibiting non-linear electrical behavior are currently well-known: thus silicon carbide (SiC) surge arrestors and zinc oxide (ZnO) resistors are routinely used to absorb unwanted voltage surges on high-voltage lines and in low-voltage electrical circuits and electronic circuits.
The electrical characteristic of a component of this kind is approximately defined by an equation of the form I=k.U.sup.n in which I is the current passing through it at an applied voltage U, n is the non-linearity coefficient representing the "slope" of the non-linearity (typically varying between 3 and 10 for an SiC resistor and between 20 and 70 for a ZnO resistor) and k is a constant defining the range of conductivity obtained.
In practice it is important to be able to obtain such characteristics not only with solid materials (sintered SiC and ZnO), but also with composite materials of a thermoplastic or thermo-hardening nature based on plastics materials, polymers, etc, in order to facilitate the fabrication by low-temperature techniques (compression molding, injection molding, extrusion, rolling, etc) of continuous or mass-produced parts, or when some degree of flexibility is required, as in applications to electrical wires and cables.
Such composite materials have been described in the literature, using SiC (French Pat. Nos. 1 260 453 and 1 363 222), ZnO (French Pat. No. 2 547 451) and various other metal oxides (U.S. Pat. No. 1,246,829). The coefficients of non-linearity obtained are in the range from 3 through 5. More recently, a company called Chomerics has marketed a flexible composite material based on silicon carbide and titanium ("CHOTRAP") with a coefficient of non-linearity in the order of 7 over a current range comprising 3 to 4 decades.
Applications of the composite materials described have been essentially limited to the production of sleeves for terminating medium and high-voltage insulated cables. A low current created by the equipotentials of the electric field produces a more favorable distribution of the field gradient, preventing breakdown at the terminations (see French Pat. Nos. 1 194 221, 1 260 453 and 1 363 222 and French patent application 2 423 036).
We are concerned here essentially with a "localized" layer in which there is no propagation effect, leading to a continuous low current defining the new and evenly distributed equipotentials.
It is an object of this document to disclose wave propagation structures (as opposed to a non-linear two-pole component) in which the non-linear medium is incorporated over the entire length of the propagation direction of an electromagnetic wave (characterizing the overvoltage disturbance) as a dielectric material.
In other words, the non-linear medium is operative in the distributed electrical elements of the structure.
Another object of this document is to disclose a structure of this kind which because of its distribution is free of stray effects (stray inductance, stray capacitance) characterizing two-pole structured components.
Another object of this document is to disclose a structure of this kind in which the non-linear medium is not operative when the applied electrical voltage is normal: in other words, this dielectric material functions normally as a conventional insulator. Only in the event of unwanted voltage surges appearing does this dielectric material conduct to "short-circuit" voltage surges either to ground or to another conductor.
A further object of this document is to disclose a structure of this kind in which the dielectric constant and the dielectic losses of the non-linear dielectric material are proportional to the applied voltage. In particular the (lossy) distributed capacitance increases to a significant degree, introducing a lowpass filter effect (RC network) and a change in the characteristic impedance .sqroot.L/C of the structure, and the corresponding reflections of the electromagnetic waves.
Another object of this document is to disclose a structure of this kind in which such non-linear effects are obtained with a dielectric and magnetic composite material, that is to say whereby the characteristics as defined hereinabove are complemented by magnetic effects with magnetic losses as described, for example, in French patent application No. 78 33385.
A final object of this document is to disclose a structure of this kind in which the voltage surge suppression effect (in the time domain) is combined with a filter effect by virtue of dielectric and/or magnetic absorption and reflection (in the frequency domain).
A structure of this kind thus functions simultaneously as a distributed voltage peak limiter and as a distributed lowpass filter (to the degree that losses increase with the frequency).
The practical benefit of the invention resides in the concept of distributed voltage surge suppression making it possible to distribute the dissipated power (voltage surges conducted to ground or to another conductor) and to eliminate the disadvantages of conventional non-linear (two-pole) components, such as inadequate response to high-speed transients.
Finally, the distribution of the non-linear effects in cable type structures introduces the new concept of integrating protection functions into the power or information transfer electrical connections, that is to say into the elements where these voltage surges and transients are generated and transmitted.
There are obvious benefits with regard to the protection of power distribution type links or signal and information distribution links against lightning strikes, against electromagnetic pulses due to nuclear explosions, against electrostatic discharges, and against voltage surges generated in the ignition distribution system of an automobile with breakdown of inductive loads.